Members of the Culex pipiens mosquito complex have a worldwide distribution and in various regions are vectors of St. Louis encephalitis, West Nile and Rift Valley fever viruses, and Wuchereria bancrofti. Two members of the complex, namely, Cx. p. pipiens and Cx. p. quinquefasciatus, are not uniform in their behavior, which is associated with variability in their vectorial capacities for diseases. These subspecies vary, in blood feeding avidity (mammal vs birds), degree of autogeny, resting behavior (endophilic vs. exophilic), vector competence for viruses and mating behavior. There is hence a need to more clearly define the population structure of members of the Cx. pipiens complex. Polytene chromosome rearrangements can serve as a tool to more clearly characterize and determine gene flow both within and between the subspecies. The overall objective of this proposal is to characterize the salivary gland polytene chromosomes of members of the Cx. p. pipens complex. The specific aims are: 1) Produce photomaps of salivary gland chromosomes of Cx. p. pipiens, Cx. p. quinquefasciatus and Cx. p. molestus and 2) Determine chromosomal homologies between Cx. p. pipiens, Cx. p. quinquefasciatus and Cx. p. molestus.